The Australian Open and Nick Kyrgios's matches have been taken over by the 'siuuu' — so what is it?

Have you caught yourself wondering what all that interminable noise at the Australian Open this year has been?

Are the crowd saying boo? Boo-urns? Or are they speaking Portuguese for some reason?

The answer is actually the latter, as the soundtrack to Nick Kyrgios's raucous matches became the uniform cry of «siuuu».

Confused? You're quite right to be. Here's a bit of background.

It all starts from Cristiano Ronaldo, one of the greatest footballers of all time.

During his time at Real Madrid early last decade, he began celebrating goals with cries of «si» — «yes» in Portuguese — which eventually morphed into a more guttural «siu».

He continued that celebration through his time with Juventus and now with Manchester United, and in that time it became somewhat of a trademark.

Opposition players and fans have adopted it on occasion too, mostly to try to get under Ronaldo's skin.

That's a really good question. There's no obvious reason for why it started at the tennis, and even Kyrgios himself said he was surprised to see it catch on.

Kyrgios did egg the crowd on after his win over Brady, replicating Ronaldo's celebration on court.

That was probably enough to ensure the «siuuu» was back in force for the Medvedev match.

They all seem to hate it, and many of them seemed to think they were being booed initially.

After his second-round win over Kyrgios, Medvedev took a light-hearted pop at the crowd and left a parting message on the camera.

"[Jeering] between first and second serve, that's where, you know, it's tough," Medvedev said.

«It's not good for the game I think to do it, because probably people don't know, but when you're getting ready for the second serve, it's a tough moment.

»I think people should respect both players and just, you